The present invention relates to an electrode for a lithium secondary battery excellent in initial charge/discharge characteristics and cycleability and to a lithium secondary battery made using the electrode.
A lithium secondary battery having, for example, a battery construction in which an electrode containing lithiated cobalt dioxide is used as a cathode and an electrode containing a carbon material is used as an anode utilizes the electrochemical reversible reaction in which lithium from the cathode is doped into the anode through an electrolyte at a charging stage, and the lithium is undoped from the anode and doped into the cathode through the electrolyte at a discharging stage. Characteristics required for the electrodes are a large doping ability (capacity) and undoping ability (capacity) and less deterioration in capacity at the doping and undoping cycle.
From these viewpoints, various proposals have been made for electrodes containing carbon materials capable of recharging lithium and for batteries using such electrodes. Specifically, there have been proposed various batteries made using as the anode a wide variety of carbon materials including graphite materials or carbon materials having a random layer structure, such as batteries made using a graphite anode containing rechargeable lithium in the crystal (Japanese Patent Kokai (Unexamined Publn.) No. 57-208079), batteries made using an anode of a graphitic material comprising graphitizable spheroidal particles (Japanese Patent Kokai (Unexamined Publn.) No. 4-115457), batteries made using an anode of carbon material having a pseudo-graphite structure prepared by carbonizing organic polymer compounds (Japanese Patent Kokai (Unexamined Publn.) No. 62-122066), batteries made using an anode of carbon material having a specific structure (Japanese Patent Kokai (Unexamined Publn.) No. 62-90863), and batteries made using an anode of carbon material having a random layer structure (Japanese Patent Kokai (Unexamined Publn.) No. 2-66856). Furthermore, as for cathode materials, there have been proposed batteries made using as a cathode a metallic chalcogen compound or a carbon material having a specific structure prepared by carbonizing an organic polymer compound (Japanese Patent Kokai (Unexamined Publn.) No. 62-122066) and batteries made using as a cathode a composite oxide comprising an alkali metal and a transition metal and additionally Al, In or Sn (Japanese Patent Kokai (Unexamined Publn.) No. 62-90863).
As a result of experiments and investigations conducted by the inventors using a wide variety of carbon powders for electrodes, it has been confirmed that among carbon powders, graphite powders are preferred as anode materials for lithium secondary batteries in view of the fact that they have a large doping and undoping ability for lithium and the voltage at which lithium is undoped is close to the lithium potential, but there is the problem that the initial charge/discharge characteristics are insufficient and batteries of a high capacity cannot be obtained.
That is, in the case of the battery construction having an electrode containing a lithium-rechargeable material and an electrode containing a carbon powder, 100% of the lithium doped in the carbon material is not undoped in the initial stage and the utilizing efficiency of lithium is low and batteries of a high capacity cannot be obtained. In other words, a large amount of initial irreversible capacity components (irreversible capacity component=charging capacity of lithium to carbon material electrode-discharging capacity of lithium from carbon material electrode) are present in the carbon materials.
Specifically, the capacity of an electrode containing a carbon material to dope and undope lithium per unit weight differs greatly depending upon the kind of the carbon material and is about 100-300 mAH/g. However, the irreversible capacity at the initial charging and discharging is also very high, namely, about 150-600 mAH/g for graphite materials and about 1000 mAH/g for carbon black and vapor-grown carbon materials made using iron catalysts. Therefore, batteries made using the electrodes containing these carbon materials are low in utilizing efficiency of lithium and batteries of a high capacity cannot be obtained.
On the other hand, electrodes containing LiCoO.sub.2 or LiNiO.sub.2 which are a composite oxide of an alkali metal and a transition metal have an irreversible capacity of about 40 mAH/g at the initial charging and discharging and the utilization efficiency of lithium is insufficient.
Furthermore, in making a sheet electrode, there are problems that bonding between the current collector and the electrode layer is insufficient and separation occurs therebetween and, as a result, fabrication of the sheet electrode is difficult, or in the case of a battery made using a sheet electrode insufficient in the bonding, the electrode layer comes off from the collector due to charging and discharging to cause degradation of the cycling behavior.